1. Field of the invention
The present invention relates, in general, to a method of the preparation of a sheet of thermoplastic composite material composition and, more particularly, to a method for preparing prepregs of thermoplastic composite resin, using a strand of or a mat of glass fibrous reinforcement, capable of providing reusability, excellent mechanical strength, superior thermal resistance and superior impregnation property to the prepregs.
2. Description of the Prior Art
Generally, fiber-reinforced plastics have employed reinforcing materials made of staples resulting from glass fibers. However, the fiber-reinforced plastics are difficult to manufacture, for example, glass fibrous strands used in the plastics are often cut in a compounding process or an injection molding process, which leads to the degradation phenomenon of impact strength in final products, and thus, the final products are not suitable to applications requiring high impact strength. In addition, when the resinous material for the fiber-reinforced plastics is injected, the injected product is poor in dimensional stability due to the orientation of the glass fibers contained the resinous material. Meanwhile, a thermosetting composite resin shows excellent thermal resistance, but typically accompanies reduction in productivity because of the requirement of its curing time and is problematic in impact resistance.
Research and development efforts have been directed to the improvement of the final molded product in mechanical strength as well as lightness. As a result of groping for useful methods for molding the fiber-reinforced thermoplastic composite resin, a stamping process has been developed which comprises heating a sheet of prepreg above melting temperature of the resin employed in the composite material, to make the sheet-like thermoplastic composite material have fluidity, subjecting the fluidized composite material to compression molding in a heated mould, and cooling the composite resin in the mould to produce a product, which is useful for a plurality of purposes, such as auto parts, sheetings for construction or civil engineering, etc.
Because when a sheet of thermoplastic composite resin comprising a non-oriented, glass fibrous mat is molded, it is required to fill the prepreg thereof in a mould, it is general to needle-punch the non-oriented, glass fibrous mat to allow the mat to be fluidizable. However, the use of both the thermoplastic resin and the needled, non-oriented fibrous mat improves mechanical properties but results in inferior surface properties. In particular, there is a problem that when a molded product made therefrom is unable to be used as an applications which is applied with only unidirectional forces due to its lack of physical properties.
Determined as the properties of the composite material are according to the shape of the glass fibrous mat, the content ratio of the glass fiber to the resin, the distribution of the glass fiber in the resin, the coagulation state between the glass fibers and the resin and the like, the degree of the impregnation of the fibrous mat in the substrate is of great importance, because poor impregnation causes void volumes which detrimentally affect the physical properties thereof.
Such a sheet of thermoplastic composite resin prepreg is disclosed in many prior arts, for example, U.S. Pat. Nos. 3,664,909, 3,684,645, 3,849,148 and 3,883,333, which also say preparation methods along with a variety of specific processings therefor. Thermoplastic composite materials with superior impact resistance and rigidity can be produced by the conventional methods. However, with regard to the impregnation of the glass fiber into resin, it is insufficient in the conventional thermoplastic composite materials produced. Further, the prior art thermoplastic composite materials have poor fluidities in molding.
In order to enhance the impregnation of glass fiber into resin, there is suggested the use of high fluidity polypropylene in EP. No. 211,249. The resulting composite material employing the high fluidity polypropylene, however, is unsatisfactory in surface properties. Besides, when compress-molding the prepreg of thermoplastic composite material into a complicate shape, the complicate shape effects poor fluidity in a final product in spite of the original high fluidity, leading to the decline of reinforcing effect.